1. Field of the Invention
The present invention relates to a single mode optical waveguide of the type in which a core glass portion is embedded in a cladding layer disposed on a substrate.
2. Description of the Prior Art
In a silica single mode optical waveguide which can be fabricated on a silica substrate or a silicon substrate, its cross section has a size which can be determined to be about 5-10 .mu.m so as to correspond to that of a conventional single mode optical fiber, so that the silica single mode optical waveguide is expected as means for realizing practical waveguide type component parts having an excellent matching characteristic with an optical fiber.
The optical waveguide of the type described is disclosed in detail in Electronics Letters, 24 Oct. 1985, Vol 21, No. 20, pp. 1020-1021, "HIGH SILICA SINGLE-MODE OPTICAL REFLECTION BENDING AND INTERSECTING WAVEGUIDES" or in Electronics Letters, 13 Mar. 1986, Vol. 22, No. 6, pp. 321-322, "LOW-LOSS HIGH-SILICA SINGLE MODE CHANNEL WAVEGUIDES".
FIG. 1 is a cross sectional view showing a structure of a conventional silica single mode optical waveguide. Reference numeral 1 designates a silica glass substrate; 2, a silica glass core portion and 3, a silica glass cladding layer surrounding the core portion 2. The cross sectional size of the core portion is about 10 .mu.m. The thickness of the cladding layer 3 is tens of micrometers. The thickness of the substrate 1 is of the order of 2 mm. Such silica single mode optical waveguide can be fabricated by a combination of deposition technique for depositing a glass film by flame hydrolysis of raw material gases such as SiCl.sub.4, TiCl.sub.4 or the like with reactive ion etching technique. For instance, reference is made to MICROOPTICS NEWS, 1986, 4/15, Vol. 4, No. 2, pp. 33(108)-38(113), "Microlithograpy of High-Silica Channel Optical Waveguides".
In the silica single mode optical waveguide of the type shown in FIG. 1, tensile stresses are imparted to the inside of the film surface of the cladding layer 3 because of the difference in thermal expansion coefficient between the cladding layer 3 and the silica glass substrate 1. That is, it exhibits stress-induced birefringence and, in general, the value of stress-induced birefringence B is of the order of 10.sup.-5.
The birefringence in an optical waveguide is one of the important factors which determines performance of a waveguide type optical component part, so that it is desirable to control the a birefringence value with a high degree of accuracy. However, in a conventional silica single mode waveguide, there is no way to vary or control the birefringence value except to change glass compositions or the kinds of substrate. In addition, the directions of the principal axes of stress are limited to the direction in parallel with the surface of the substrate and to the direction perpendicular thereto. In addition, it is difficult to vary locally the birefringence properties in the optical waveguides. Therefore, these problems constitute obstacles in case of fabricating a waveguide type optical component which exhibits a high degree of performance.
In some case, instead of silica glass, silicon is used as the substrate 1. In this case, the cross sectional size of the core portion 2 is also about 10 .mu.m. The thickness of the cladding layer 3 is of the order of 50 .mu.m. The thickness of the silicon substrate is of the order of 0.4 through 1 mm.
In the case of a silica single mode optical waveguide fabricated on a silicon substrate, a strong compression stress of the order of 15 kg/mm.sup.2 is applied to the interior of the glass film surface due to the difference in thermal expansion coefficient between the silica glass and the silicon substrate, so that the optical waveguide exhibits stress-induced birefringence. Birefringence of an optical waveguide is one of the important factors which determine performance of a waveguide type optical component part. Therefore, it is desired that the birefringence be controlled with a high degree of accuracy. In general, the value of birefringence is of the order of 10.sup.-4, so that it is difficult to remove the adverse effects of stress from the silicon substrate. This problem also constitutes an obstacle in the case of fabrication of a waveguide type optical component.
Furthermore, in the case of a structure as shown in FIG. 1, in which a core portion 2 constituting a main body of an optical waveguide is placed in intimate contact with a silicon substrate 1 via a cladding layer 3, it is completely impossible to mechanically move the main body of the optical waveguide on the substrate. This problem also constitutes an obstacle when a variety of functions can be realized by an optical waveguide.
Meanwhile, as to the structure of optical fibers, there is disclosed in, for instance, Journal of Lightwave Technology, Vol. LT-1, No. 1, March 1983, pp. 38-43, "Fabrication of Polarization-Maintaining and Absorption-Reducing Fibers", an optical fiber which maintains its polarization properties by providing a stress applying portion surrounding a core portion in a cladding layer. In the optical fiber, however, it is impossible to locally adjust the stress in the longitudinal direction of the waveguide.
In an optical circuit disclosed in Laid-Open Japanese Patent Application No. 196,204/1982, stress birefringence is adjusted by varying the width of a ridge in a YIG optical waveguide (ridge type) on a GGG substrate to coincide the transmission phase constant of a TE wave with that of a TM wave. If, however, the ridge width is varied, the structure of the core portion is varied, so that the spot size of the transmitted light is the also varied. That is, birefringence cannot be adjusted independently of the core structure.
Laid-Open Japanese Patent Application No. 4,022/1982 discloses a method for producing stress-induced birefringence in a ridge type optical waveguide by loading a dielectric film (SiO.sub.2 film) on a ridge type YIG optical waveguide. If, however, the dielectric film is loaded on the ridge type optical waveguide, not only the stress-induced birefringence but also the structure of the core portion itself are considerably varied. Therefore, as in the case of Laid-Open Japanese Patent Application No. 196,204/1982, the birefringence cannot be adjusted independently of the structure of the core portion.